1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery and a producing method therefor. In the nonaqueous electrolyte secondary battery of the invention, a grooved portion is provided in the upper portion of the outer casing thereof to protrude toward the inner portion of the outer casing in which a group of electrodes is housed, while a positive electrode and a negative electrode facing each other through a separator in the group of electrodes are wound to form a spiral, and a sealing member is placed on the upper portion of the grooved portion by means of insulated packing to seal the opening of the outer casing in an airtight manner.
2. Description of Prior Art
Nonaqueous electrolyte secondary batteries, typified by light lithium secondary batteries with high energy density, have come to be used as a power source for portable information terminals such as portable telephones and Personal Digital Assistants (PDAs), notebook personal computers, and digital cameras. A lithium secondary battery is typically produced by dissolving a negative electrode active material containing natural graphite, a binder, and the like in an organic solvent to form slurry or paste, and the slurry or paste is applied to a negative electrode core body made of copper foil or the like to produce a negative electrode. On the other hand, organic solvent, a carbon conductive material, and a positive electrode active material containing LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and the like are mixed together to form slurry or paste, and the slurry or paste is applied to a positive electrode core body made of aluminum foil or the like to produce a positive electrode.
After the positive electrode and the negative electrode are superposed on each other while sandwiching a separator formed by a polyethylene fine porous film therebetween, the positive electrode, the negative electrode and the separator are wound together by a winding apparatus, and the outermost periphery thereof is secured with tape to form a spiral electrode group. Then, after the spiral electrode group is inserted into an outer casing, a negative electrode lead (negative electrode collector tab) connected to the negative electrode and a inner bottom surface of the outer casing are connected to each other, and a positive electrode lead (positive electrode collector tab) connected to the positive electrode and a positive electrode terminal are connected to each other. A nonaqueous electrolytic solution is then injected into the outer casing, which is sealed in an airtight manner to produce the lithium secondary battery. In the nonaqueous electrolytic solution, electrolytic salt is added to the organic solvent (for example, a mixture solvent containing ethylene carbonate (EC) and dietyl carbonate (DEC)).
In recent years, the demand for lithium secondary batteries with higher capacity has increased because of improvement in the performance of electronic instruments. For example, higher capacity can be achieved by improving the packing density of the electrode active material layer. Such method however adversely affects the ability of the nonaqueous electrolytic solution to penetrate the active material layer or be impregnated therein. In other words, the nonaqueous electrolytic solution does not spread widely in the electrode, which causes deterioration in battery characteristics.
The decrease in volume of the nonaqueous electrolytic solution to penetrate or impregnate the active material layer leads to decrease in productivity, as to worsen production yield of the battery. Further, in this kind of lithium secondary battery, disk-shaped circular insulating plates provided with a hole approximately in the center portion thereof are usually arranged at the upper and lower end faces of the group of electrodes. Because the hole occupies a relatively small area of the circular insulating plate, the entire upper end face of the group of electrodes is substantially covered by the circular insulating plate, which obstructs the movement of the nonaqueous electrolytic solution, thereby further diminishing the degree of penetration or impregnation of the active material layer by the nonaqueous electrolytic solution.
For example, Japanese Laid-Open Patent Nos. H9-283111 and H9-283112 proposed to address the above problems. Japanese Laid-Open Patent No. H9-283111 proposed that the impregnation property of the nonaqueous electrolytic solution can be improved by arranging in the upper portion of the group of electrodes housed in the outer casing an insulating plate having at least one cutout portion in an outer periphery thereof.
In Japanese Laid-Open Patent No. H9-283112, the insulating plate, which has a center plane portion, an annular wall formed near the outer periphery of such center plane portion, and a collar portion formed in the outer periphery of the annular wall, is arranged in the upper portion of the group of electrodes housed in the outer casing. The center plane portion has a through-hole, through which the electrode lead is inserted in the middle of the center plane portion, and the center plane portion also has several through-holes in the periphery thereof. The collar portion has a number of through-holes, and the bottom surface of the collar portion is located on the side where the group of electrodes resides at the bottom surface of the center plane portion. The bottom surface of the annular wall is located in the same plane as the bottom surface of the center plane portion, or the bottom surface of the annular wall is located at the midpoint between a bottom surface portion of the collar portion and the bottom surface portion of the center plane portion. Pursuant to this structure, the time for infusing the nonaqueous electrolytic solution can be shortened.
However, using the means proposed in either Japanese Laid-Open Patent No. H9-283111 or No. H9-283112 does not bring about any improvement in the impregnation ability of the nonaqueous electrolytic solution, while actually causing diminution in battery characteristics and decrease in production yield. In the recent type of lithium secondary batteries, a center pin is arranged to be inserted in a hollow area formed in the central axis region of the spiral electrode group to prevent deformation of the hollow area due to charging and discharging and to ensure the passage of gas exhaust.
However, there was caused the problem that the center pin arranged to be inserted in the hollow portion formed in the central axis region of the spiral electrode group protrudes by pressing force from the spiral electrode group. To avoid this problem, the circular insulating plate, which has not been provided with a hole at the center thereof, is used. As shown in FIG. 5, in a circular insulating plate 30 with no hole, a portion 30a corresponding to the central axis region of the spiral electrode group is formed so as to cover the same, and a semicircular arced opening 32 is provided around the portion 30a. 
When the portion 30a corresponding to the central axis region of the spiral electrode group is formed such as to cover the same, the upper portion of the center pin arranged to be inserted in the hollow area formed in the middle of the spiral electrode group is covered with the portion 30a of the circular insulating plate 30 corresponding to the central axis region of the spiral electrode group as shown in FIG. 6, so that the center pin will not protrude.
However, when the upper portion of the hollow area formed in the central axis region of the spiral electrode group is covered with the portion 30a of the circular insulating plate 30 corresponding to the central axis region of the spiral electrode group, a path for the nonaqueous electrolytic solution to penetrate the central axis region of the spiral electrode group cannot be ensured during injection of the nonaqueous electrolytic solution. Accordingly, the process of penetrating each electrode in the group of electrodes takes enormous time, and the injection property of the nonaqueous electrolyte solution deteriorates, which worsens production yield while at the same time causing productivity to be diminished in this kind of nonaqueous electrolytic solution battery.
In view of the foregoing, the invention has been conceived to provide a nonaqueous electrolyte secondary battery having excellent battery performance and excellent productivity and a producing method therefor with the use of a circular insulating plate which prevents the injection property of the nonaqueous electrolytic solution from being adversely affected.